A Submillimetre Study of Nearby Star Formation using Molecular Line Data

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Date

2013-10-22

Author

Drabek-Maunder, Emily R.

Date issued

2013-10-22

Type

Thesis or dissertation

Language

en

Publisher

University of Exeter

Abstract

This thesis primarily uses submillimetre molecular line data from HARP, a heterodyne array on the James Clerk Maxwell Telescope (JCMT), to further investigate star formation in the Ophiuchus L1688 cloud. HARP was used to observe CO J = 3-2 isotopologues: 12CO, 13CO and C18O; and the dense gas tracer HCO+ J = 4-3.
A method for calculating molecular line contamination in the SCUBA-2 450 and 850 μm dust continuum data was developed, which can be used to convert 12CO J =6-5and J =3-2 maps of integrated intensity (K km s−1) to molecular line flux (mJy beam−1) contaminating the continuum emission. Using HARP maps of 12CO J = 3-2, I quantified the amount of molecular line contamination found in the SCUBA-2 850 μm maps of three different regions, including NGC 1333 of Perseus and NGC 2071 and NGC 2024 of Orion B. Regions with ‘significant’ (i.e. > 20%) molecular line contamination correspond to molecular outflows. This method is now being used to remove molecular line contamination from regions with both SCUBA-2 dust continuum and HARP 12CO map coverage in the Gould Belt Legacy Survey (GBS).
The Ophiuchus L1688 cloud was observed in all three CO J = 3-2 isotopologues. I carried out a molecular outflow analysis in the region on a list of 30 sources from the Spitzer ‘c2d’ survey [Evans et al., 2009]. Out of the 30 sources, 8 had confirmed bipolar outflows, 20 sources had ‘confused’ outflow detections and 2 sources did not have outflow detections.
The Ophiuchus cloud was found to be gravitationally bound with the turbulent kinetic energy a factor of 7 lower than the gravitational binding energy. The high-velocity outflowing gas was found to be only 21% of the turbulence in the cloud, suggesting outflows are significant but not the dominant source of turbulence in the region. Other factors were found to influence the global high-velocity outflowing gas in addition to molecular outflows, including hot dust from nearby B-type stars, outflow remnants from less embedded sources and stellar winds from the Upper Scorpius OB association.
To trace high density gas in the Ophiuchus L1688 cloud, HCO+ J = 4-3 was observed to further investigate the relationship between high column density and high density in the molecular cloud. Non-LTE codes RADEX and TORUS were used to develop density models corresponding to the HCO+ emission. The models involved both constant density and peaked density profiles. RADEX [van der Tak et al., 2007] models used a constant density model along the line-of-sight and indicated the HCO+ traced densities that were predominantly subthermally excited with den- sities ranging from 10^3–10^5 cm^−3. Line-of-sight estimates ranged from several parsecs to 90 pc, which was unrealistic for the Ophiuchus cloud. This lead to the implementation of peaked density profiles using the TORUS non-LTE radiative transfer code. Initial models used a ‘triangle’ density profile and a more complicated log-normal density probability density function (PDF) profile was subsequently implemented. Peaked density models were relatively successful at fitting the HCO+ data. Triangle models had density fits ranging from 0.2–2.0×10^6 cm^−3 and 0.1–0.3×10^6 cm^−3 for the 0.2 and 0.3 pc cloud length models re- spectively. Log-normal density models with constant-σ had peak density ranges from 0.2–1.0 ×10^5 cm^−3 and 0.6–2.0×10^5 cm^−3 for 0.2 and 0.3 pc models respectively. Similarly, log-normal models with varying-σ had lower and upper density limits corresponding to the range of FWHM velocities. Densities (lower and upper limits) ranged from 0.1–1.0 ×10^6 and 0.5–3.0 ×10^5 cm^-3 for the 0.2 and 0.3 pc models respectively.
The result of the HCO+ density modelling indicated the distributions of starless, prestellar and protostellar cores do not have a preference for higher densities with respect to the rest of the cloud. This is contrary to past research suggesting the probability of finding a submillimetre core steeply rises as a function of column density (i.e. density; Belloche et al. 2011; Hatchell et al. 2005). Since the majority of sources are less embedded (i.e Class II/III), it is possible the evolutionary state of Ophiuchus is the main reason the small sample of Class 0/I protostars do not appear to have a preference for higher densities in the cloud.